Despite new vaccines, Neisseria meningitidis, especially serogroup B, remains a leading cause of meningitis and rapidly fatal sepsis, usually in otherwise healthy individuals. The regulatory mechanisms of the major virulence factors of this devastating human pathogen are still not well-characterized. The long-term goal of this research is to elucidate the regulatory mechanisms of virulence determinants in meningococcal pathogenesis and provide a fuller knowledge base for the development of preventative and therapeutic interventions. The specific and novel hypothesis is that host environment-initiated reductive and contact signals activate the global regulator MisRS two-component regulatory system to differentially mediate expression of important virulence determinants. Exciting new preliminary data show the involvement of MisRS in host cell attachment and invasion, in regulation of capsule expression as well as the initial identification of a host signal mimic: dithiothreitol (DTT) stress. Building on these results and the newly discovered role of MisRS in controlling the contact-regulatory promoter element of Neisseria (CREN) that coordinately up-regulates gene products after host cell contact, we will define the inducing nature of host signals and the signal transduction mechanism of MisS sensor kinase, develop a molecular understanding of the regulatory mechanisms of MisR on capsule expression, and further delineate the role of the MisRS system in host-pathogen interactions. In Specific Aim I, the global transcription and protein expression profiles upon DTT treatment will be determined, contrasting to profiles caused by non-inducing reductants, to delineate the specific inducing nature of DTT stress. We will also define the sensing domain of MisS and examine whether (i) protein misfolding, (ii) redox imbalance or (iii) protein sulfenylation contributes to the inductio of MisRS, thus demonstrating the sensing capability of MisS and advancing our understanding on the nature of the host microenvironment mimicked by DTT that signals N. meningitidis for rapid adaptation. Further, substrates of the Dsb reducing branch and the contribution of the MisRS/Dsb systems in adhesin maintenance and interactions with epithelial cells will be defined, thereby providing broader knowledge of the overall importance of the disulfide-folding pathway in meningococcal pathogenesis. In Specific Aim II, we will define which genetically distinguishable cell contact event leads to the activation of MisS and the CREN regulon. In conjunction, the role of MisRS in the cell contact-mediated regulation of capsule expression will be characterized through probing the in vitro and in vivo interactions of MisR with all cps promoters. The differential regulatory mechanisms of target genes upon detecting the reductive and cell contact signals will also be investigated. Together, the studies will shed new light on th novel signal perception mechanisms of MisS that govern self-activation and the regulation of critical virulence determinants, and build a better understanding of the pathogenesis of N. meningitidis.